[time-nuts] A look inside the DS3231

Didier Juges shalimr9 at gmail.com
Sun Jul 30 09:53:51 EDT 2017

That device also has analog circuitry for the oscillator itself and the
temperature sensor and the temperature compensation.
I believe I have read an app note some time ago, it may have been from
Maxim describing a kind of ring oscillator being used as a temperature
sensor which drew much less power than a bandgap or a PN junction and
directly produced a digital output.
The DAC itself, or whatever circuit they use for temp compensation also has
analog components and must use pico power.
Quite amazing.

On Jul 30, 2017 7:13 AM, "Attila Kinali" <attila at kinali.ch> wrote:

> On Sun, 30 Jul 2017 12:23:17 +0200
> Pete Stephenson <pete at heypete.com> wrote:
> > > > - I find it remarkable that this circuit can operate on less than a
> > > > microamp during normal usage, including temperature conversion.
> > >
> > > That's not so remarkable. If you make the transistors long, then
> > > you get very low leakage. Couple that with small clock frequency
> > > and you use very little current. Modern ICs only use so much current
> > > because they have so many transistors, which are also optimized
> > > for being fast, rather then low leakage.
> >
> > Good point! I admit the details of optimizing transistors for different
> > purposes is beyond my ken, and I appreciate the insight.
> There are multiple optimization points. One is to select a prodcution
> process that is optimized for low leakage. I.e. thick gate oxide
> and high threshold voltage. Both of these parameters imply higher
> suplly voltage.
> Then, in the design, you make your transistors long and large.
> The problem here is, that power consumption scales proportional
> to the square of supply voltage, the gate capacitance and the
> switching frequency. This means, if you choose a low leakage
> process, and thus high supply voltage, your power consumtion
> will go up. The same goes for choosing large transistors.
> Hence it becomes a trade-off between static (leakage) and
> dynamic (through gate capacitance) power consumption.
> > > > The DS3231 has on-board temperature monitoring to correct the crystal
> > > > frequency: is this something where they would have bothered putting a
> > > > separate sensor next to the crystal itself, or are the die and the
> > > > crystal are close enough and in the same package that they could use
> an
> > > > on-die sensor like a diode and call that "good enough"?
> > >
> > > My guess would be that it's a PN-junction or a bandgap temperature
> > > sensor somewhere on the chip. Adding another part increases the cost
> > > of production quite considerably.
> >
> > Indeed. At first glance, I was surprised not to see tiny discrete
> > capacitors within the chip package itself, as I assumed (incorrectly)
> > that getting sufficient capacitance to steer a crystal a little would
> > require larger capacitors than could be easily put on a die, but then I
> > remembered that each LSB in the aging register only changes the
> > frequency by 0.1ppm at 25C, so that wouldn't need a large amount of
> > capacitance.
> As a rule of thumb, you can assume that in an "old" (aka large node size)
> process the gate capacitance is approximately 1nF per mm^2. So, you can
> build quite easily 10-100pF of capacitors on-chip.
>                         Attila Kinali
> --
> You know, the very powerful and the very stupid have one thing in common.
> They don't alters their views to fit the facts, they alter the facts to
> fit the views, which can be uncomfortable if you happen to be one of the
> facts that needs altering.  -- The Doctor
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